This invention relates generally to casting and, more particularly, to systems and methods for pressure casting parts using a supported shell mold.
Pressure casting is a known technique that when used with certain alloys can produce desirable properties. However, a part having re-entrant features, which are undercut features positioned perpendicular to the molding pressure axis, could not be pressure cast in the ordinary way because the presence of re-entrant features would prevent the removal of the cast part from the permanent die molds. Instead, parts including re-entrant features were typically cast by gravity pouring or vacuum flowing liquid metal into a plaster or ceramic mold. Parts formed in this manner may lack the mechanical properties of high-pressure, permanent mold casting.
Chandley et al., in U.S. Pat. No. 5,069,271, describe an example of one such process. The Chandley et al. process involves applying a vacuum to an investment-type mold to draw molten metal into the mold in a counter-gravity casting process. As with all casting processes of this type, however, this process is conducted at fairly low pressures, which leads to porosity and shrinkage as the molten metal solidifies. As a result, this type of process lacks the mechanical property capabilities of high-pressure, permanent mold casting.
Furthermore, known pressure casting processes utilize turbulent filling of the mold cavity, which leads to formation of oxide-type defects and gas porosity in the cast parts. Ultimately, these defects negatively impact the fatigue properties of the resulting parts. Moreover, known pressure casting processes are useful with only a limited selection of alloys due to the tendency of certain alloys to solder to permanent molds.
The present invention solves one or more of the problems associated with the methods of the prior art and combines the benefits of high-pressure, permanent mold casting with the flexibility of being able to create a wide array of parts, including those with re-entrant features.
One aspect of the present invention includes a method of casting. This method includes investing a shell mold around a pattern fabricated from an expendable material and then removing the expendable material from the shell mold. The shell mold is located within a housing such that an inlet port of the shell mold communicates with an opening in the housing. A supporting material is provided and substantially fills an open volume between an external surface of the shell mold and an interior surface of the housing. A molten material is then pressure cast through the inlet port and into the shell mold.
A second aspect of the present invention includes a casting system. This system includes a pressure casting apparatus having an inlet sprue and a die cavity. A mold assembly is configured to fit within the die cavity. The mold assembly includes a housing including an interior volume and an opening through a wall of the housing. A refractory shell mold is disposed within the interior volume of the housing. The refractory shell mold includes an internal mold cavity and has an inlet port that communicates with the opening in the housing and mates with the inlet sprue. A supporting material substantially fills a volume between an external surface of the refractory shell mold and an interior surface of the housing.